1. Field of the Invention
The present invention relates to a double-tuned circuit used as an inter-stage tuning circuit of a television-set tuner, and more specifically, to a double-tuned circuit in which a high-band coupling coil and a low-band coupling coil are independently provided to couple a primary tuning circuit with a secondary tuning circuit.
2. Description of the Related Art
FIG. 5 shows a conventional double-tuned circuit. In FIG. 5, a double-tuned circuit 51 is connected to a high-frequency amplifier (not shown) at an input end 51a and to a mixer (not shown) at an output end 51b.
The double-tuned circuit 51 is switched between a high band and a low band by a so-called band switching method.
A primary tuning circuit 51c is provided with a high-band tuning coil 52 and a low-band tuning coil 53 connected in series by one end of each coil. Between the connection point thereof and the ground, a switch diode 54 and a DC-blocking capacitor 55 connected in series are connected.
The cathode of the switch diode 54 is connected to the connection point of the high-band tuning coil 52 and the low-band tuning coil 53, and the other end of the high-band tuning coil 52 is connected to the input end 51a.
Between the input end 51a and the ground, a DC-blocking capacitor 56 and a varactor diode 57 connected in series are connected, and the anode of the varactor diode 57 is grounded.
A secondary tuning circuit 51d is provided with a high-band tuning coil 58 and a low-band tuning coil 59 connected in series by one end of each coil. Between the connection point thereof and the ground, a switch diode 60 and a DC-blocking capacitor 61 connected in series are connected.
The cathode of the switch diode 60 is connected to the connection point of the high-band tuning coil 58 and the low-band tuning coil 59, and the other end of the high-band tuning coil 58 is connected to the output end 51b.
Between the output end 51b and the ground, a DC-blocking capacitor 62 and a varactor diode 63 connected in series are connected, and the anode of the varactor diode 63 is grounded.
The other end of the low-band tuning coil 53 in the primary tuning circuit 51c is connected to the other end of the low-band tuning circuit 59 in the secondary tuning circuit 51d. Between the connection point thereof and the ground, a coupling coil 64 and a DC-blocking capacitor 65 connected in series are provided. The DC-blocking capacitor 65 is connected to the ground.
The connection point of the coupling coil 64 and the DC-blocking capacitor 65 is connected to one end of a feed resistor 66, and the other end thereof is connected to a low-band switching terminal 67.
The connection point of the anode of the switch diode 54 and the DC-blocking capacitor 55 in the primary tuning circuit 51c is connected to one end of a feed resistor 68, and the other end thereof is connected to a high-band switching terminal 69. In the same way, the connection point of the anode of the switch diode 60 and the DC-blocking capacitor 61 in the secondary tuning circuit 51d is connected to one end of a feed resistor 70, and the other end thereof is connected to the high-band switching terminal 69.
The connection point of the DC-blocking capacitor 56 and the varactor diode 57 in the primary tuning circuit 51c is connected to one end of a feed resistor 71, and the other end thereof is connected to a tuning voltage terminal 72. In the same way, the connection point of the DC-blocking capacitor 62 and the varactor 63 in the secondary tuning circuit 51d is connected to one end of a feed resistor 73, and the other end thereof is connected to the tuning voltage terminal 72.
In the above configuration, a tuning voltage is applied to the tuning voltage terminal 72. The voltage is changed to alter the capacitance of the varactor diode 57 in the primary tuning circuit 51c and the capacitance of the varactor diode 63 in the secondary tuning circuit 51d, and thereby the tuning frequency of the double-tuned circuit 51 is to be changed.
Band switching will be described below, in which the double-tuned circuit 51 is switched between a state in which a high-band television signal is received and a state in which a low-band television signal is received.
To switch the double-tuned circuit 51 shown in FIG. 5 to a state in which a low-band television signal is received, a band switching voltage of, for example, 5 V is applied to the low-band switching terminal 67. A voltage is applied to the switch diode 54 and the switch diode 60 in the reverse directions and both switch diodes 54 and 60 become a non-continuity state. Therefore, the double-tuned circuit 51 shown in FIG. 5 works as an equivalent circuit shown in FIG. 6.
In the equivalent circuit shown in FIG. 6, a coil 74 in the primary tuning circuit 51c indicates the high-band tuning coil 52 and the low-band tuning coil 53 connected in series, and a coil 75 in the secondary tuning circuit 51d indicates the high-band tuning coil 58 and the low-band tuning coil 59 connected in series.
The coupling coil 64 connected between the ground and the connection point of the coil 74 in the primary tuning circuit 51c and the coil 75 in the secondary tuning circuit 51d determines the coupling state between the primary tuning circuit 51c and the secondary tuning circuit 51d at a low band. The inductance thereof is specified in advance such that the double-tuned circuit 51 has a predetermined transfer characteristic. Therefore, the tuning coil 74 in the primary tuning circuit 51c and the tuning coil 75 in the secondary tuning circuit 51d do not cause direct inductive coupling.
On the other hand, to switch the double-tuned circuit 51 shown in FIG. 5 to a state in which a high-band television signal is received, a band switching voltage of, for example, 5 V is applied to the high-band switching terminal 69. A voltage is applied to the switch diode 54 and the switch diode 60 in the forward directions and both switch diodes 54 and 60 become a continuity state.
As a result, the connection point of the high-band tuning coil 52 and the low-band tuning coil 53 in the primary tuning circuit 51c, and the connection point of the high-band tuning coil 58 and the low-band tuning coil 59 in the secondary tuning circuit 51d are ground in high frequencies. Since the low-band tuning coil 53 in the primary tuning circuit 51c, the low-band tuning coil 59 in the secondary tuning circuit 51d, or the coupling coil 64 does not function, the double-tuned circuit 51 shown in FIG. 5 works as an equivalent circuit shown in FIG. 7.
Therefore, the high-band tuning coil 52 in the primary tuning circuit 51c and the high-band tuning coil 58 in the secondary tuning circuit 51d are disposed at a predetermined distance to be coupled to obtain a coupling state between the primary tuning circuit 51c and the secondary tuning circuit 51d.
In the conventional double-tuned circuit 51, however, the positional relationship (layout) between the high-band tuning coil 52 in the primary tuning circuit 51c and the high-band tuning coil 58 in the secondary tuning circuit 51d needs to be specified in advance to obtain an appropriate coupling state between the primary tuning circuit 51c and the secondary tuning circuit 51d in a state in which a high-band television signal is received. Therefore, the degree of freedom in designing the layout is limited.